Soy-based proteins are core ingredients in rainbow trout (Oncorhynchus mykiss) diets. The goal of the study was to increase the usage of soybean meal (SBM), decrease the soy protein concentrate (SPC) in trout feeds and determine the strain-specific resistance against bacterial and viral pathogens. The study was composed of two phases, phase I: pre-challenge study, fish feeding trial for growth performance followed by phase II, a disease challenge trial – viral (Infectious hematopoietic necrosis virus, IHNV) and bacterial (Flavobacterium psychrophilum, Fp) pathogens.
Phase I: In 8 weeks feeding trial, 1500 fish (4.2 g) were distributed into 10 treatments in triplicates (50 fish/tank) following a 2 ´ 5 factorial design, where two types of rainbow trout strains, i.e., commercial strain 1 (CT1) vs. commercial strain 2 (CT2) were fed with 5 different isonitrogenous (44% crude protein) diets with varying ratios of SBM to SPC: Diet 1 (control), 0% SBM/SPC + 30% fishmeal; Diet 2, 11% SBM + 18% SPC; Diet 3, 22% SBM + 13% SPC; Diet 4, 33% SBM + 6% SPC; and Diet 5, 44% SBM + 0% SPC. Phase II: After 8 weeks of feeding trial, fish from their respective treatments were combined and split into two groups randomly. One group was challenged with IHNV 220-90 by i.p. with 4000 pfu/fish, the second group was challenged with Fp by i.m. with 1.21 x 107 cfu/fish.
Based on the growth performance, increasing the SBM up to 33% with a reduced SPC of 6% (Diet 4) showed no significant (p > 0.05) difference with the control (Diet 1). For the CT1 with IHNV challenge, fish fed diet 3 had the highest Cumulative Percent Mortality (CPM) at 17.78%±0.12, while the fish with diet 5 had the lowest CPM at 13.33%±0.07, but the IHNV CPMs did not show significant difference among treatments (p<0.05). For the CT1 with Fp challenge, fish with diet 3 had the highest CPM at 46.67%±0.12, while the fish with diet 1 had the lowest CPM at 22.22%±0.21, but the Fp CPMs did not show a significant difference among treatments (p<0.05). Similar results were observed in the CT2 challenges, no significant difference among treatments. When comparing the same diet between the two strains, a significant difference was observed between CT1-Fp-C (46.67%±0.12) and CT2-Fp-C (15.56%±0.04) (p=0.0326). A significant difference was also observed between CT1-Fp-D (28.89%±0.04) and CT2-Fp-D (11.11%±0.04) (p=0.0048). No significant difference was found for the other diet between the two strains. Antibody responses to IHNV or Fp were detected at 7 and 28 dpi in fish exposed to the respective pathogens; however, there were no significant differences among different diet treatments for both strains CT1 and CT2. The results revealed that different fish strains fed different diets did not show a significant difference in lysozyme activity.
The distal intestinal histology is being analyzed. Genes associated with gut barrier integrity, acute inflammatory-related cytokines and chemokines, NF-kB and TNF-α-related genes, and regulators of B and T lymphocyte function are being analyzed. Overall, CT2 strain performed better than CT1 strain in terms of growth performance and disease resistance against pathogens.